Method and system for a reference symbol (RS) frequency control loop for TCXO synchronization and tracking

ABSTRACT

Aspects of a method and system for an RS frequency control loop for TCXO synchronization and tracking may include tracking a carrier frequency in an Orthogonal Frequency Division Multiplexing (OFDM) signal based on at least a reference symbol set. A receiver frequency may be adjusted based on at least the tracked carrier frequency. The carrier frequency may be tracked by generating an output signal as a function of a frequency offset Δf in a frequency discrimination feedback loop. The reference symbol (RS) set may be generated in an RS extraction module or circuit, from at least a fast Fourier transform of the received OFDM signal. The receiver frequency may be coarsely adjusted and then finely adjusted. The coarse receiver frequency adjustment may be based on processing at least a primary synchronization signal and a secondary synchronization signal.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference to:

-   U.S. application Ser. No. 12/184,353, filed on Aug. 1, 2008; and-   U.S. application Ser. No. 12/184,383, filed on Aug. 1, 2008.

Each of the above referenced applications is hereby incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to signal processing forcommunication systems. More specifically, certain embodiments of theinvention relate to a method and system for a reference symbol (RS)frequency control loop for TCXO synchronization and tracking.

BACKGROUND OF THE INVENTION

Mobile communications have changed the way people communicate and mobilephones have been transformed from a luxury item to an essential part ofevery day life. The use of mobile phones is today dictated by socialsituations, rather than hampered by location or technology. While voiceconnections fulfill the basic need to communicate, and mobile voiceconnections continue to filter even further into the fabric of every daylife, the mobile Internet is the next step in the mobile communicationrevolution. The mobile Internet is poised to become a common source ofeveryday information, and easy, versatile mobile access to this datawill be taken for granted.

Third generation (3G) cellular networks have been specifically designedto fulfill these future demands of the mobile Internet. As theseservices grow in popularity and usage, factors such as cost efficientoptimization of network capacity and quality of service (QoS) willbecome even more essential to cellular operators than it is today. Thesefactors may be achieved with careful network planning and operation,improvements in transmission methods, and advances in receivertechniques. To this end, carriers need technologies that will allow themto increase throughput and, in turn, offer advanced QoS capabilities andspeeds that rival those delivered by cable modem and/or DSL serviceproviders. Recently, advances in multiple antenna technology and otherphysical layer technologies have started to significantly increaseavailable communications data rates.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A method and/or system for a reference symbol (RS) frequency controlloop for TCXO synchronization and tracking, substantially as shown inand/or described in connection with at least one of the figures, as setforth more completely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a diagram illustrating exemplary cellular multipathcommunication between a base station and a mobile computing terminal, inconnection with an embodiment of the invention.

FIG. 1B is a diagram illustrating an exemplary MIMO communicationsystem, in accordance with an embodiment of the invention.

FIG. 2 is a diagram illustrating an exemplary OFDM symbol stream, inaccordance with an embodiment of the invention.

FIG. 3 is a diagram of an exemplary OFDM frequency acquisition andtracking system, in accordance with an embodiment of the invention.

FIG. 4 is a flow chart illustrating an exemplary frequency acquisitionand tracking process, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention may be found in a method and systemfor a reference symbol (RS) frequency control loop for TCXOsynchronization and tracking. Aspects of the method and system for an RSfrequency control loop for TCXO synchronization and tracking maycomprise tracking a carrier frequency in an Orthogonal FrequencyDivision Multiplexing (OFDM) signal based on at least a reference symbolset. A receiver frequency may be adjusted based on at least the trackedcarrier frequency.

The carrier frequency may be tracked by generating an output signal as afunction of a frequency offset Δf in a frequency discrimination feedbackloop. The reference symbol (RS) set may be generated in an RS extractionmodule or circuit, from at least a fast Fourier transform of thereceived OFDM signal. The receiver frequency may be coarsely adjustedand then finely adjusted. The coarse receiver frequency adjustment maybe based on processing at least a primary synchronization signal and asecondary synchronization signal. The reference symbol set may comprisea plurality of time-frequency slots. The plurality of time-frequencyslots may change according to a time-frequency shift and PN sequencethat may modulate the reference symbols. The PN generated sequences maybe determined by base station identifier. This base station identifiermay be determined by the primary synchronization signal (PSS) andsecondary synchronization signal (SSS). The OFDM signal may conform to aUniversal Mobile Telecommunications Standards (UMTS) long-term evolution(LTE) signal. The adjustment of the receiver frequency may be controlledvia an oscillator.

FIG. 1A is a diagram illustrating exemplary cellular multipathcommunication between a base station and a mobile computing terminal, inconnection with an embodiment of the invention. Referring to FIG. 1A,there is shown a building 140 such as a house or office, a mobileterminal 142, a factory 124, a base station 126, a car 128, andcommunication paths 130, 132 and 134.

The base station 126 and the mobile terminal 142 may comprise suitablelogic, circuitry and/or code that may be enabled to generate and processMIMO communication signals.

Wireless communications between the base station 126 and the mobileterminal 142 may take place over a wireless channel. The wirelesschannel may comprise a plurality of communication paths, for example,the communication paths 130, 132 and 134. The wireless channel maychange dynamically as the mobile terminal 142 and/or the car 128 moves.In some cases, the mobile terminal 142 may be in line-of-sight (LOS) ofthe base station 126. In other instances, there may not be a directline-of-sight between the mobile terminal 142 and the base station 126and the radio signals may travel as reflected communication pathsbetween the communicating entities, as illustrated by the exemplarycommunication paths 130, 132 and 134. The radio signals may be reflectedby man-made structures like the building 140, the factory 124 or the car128, or by natural obstacles like hills. Such a system may be referredto as a non-line-of-sight (NLOS) communications system.

Signals communicated by the communication system may comprise both LOSand NLOS signal components. If a LOS signal component is present, it maybe much stronger than NLOS signal components. In some communicationsystems, the NLOS signal components may create interference and reducethe receiver's performance. This may be referred to as multipathinterference. The communication paths 130, 132 and 134, for example, mayarrive with different delays at the mobile terminal 142. Thecommunication paths 130, 132 and 134 may also be differently attenuated.In the downlink, for example, the received signal at the mobile terminal142 may be the sum of differently attenuated communication paths 130,132 and/or 134 that may not be synchronized and that may dynamicallychange. Such a channel may be referred to as a fading multipath channel.A fading multipath channel may introduce interference but it may alsointroduce diversity and degrees of freedom into the wireless channel.Communication systems with multiple antennas at the base station and/orat the mobile terminal, for example MIMO systems, may be particularlysuited to exploit the characteristics of wireless channels and mayextract large performance gains from a fading multipath channel that mayresult in significantly increased performance with respect to acommunication system with a single antenna at the base station 126 andat the mobile terminal 142, in particular for NLOS communicationsystems. Furthermore, Orthogonal Frequency Division Multiplexing (OFDM)systems may be suitable for wireless systems with multipath.

FIG. 1B is a diagram illustrating an exemplary MIMO communicationsystem, in accordance with an embodiment of the invention. Referring toFIG. 1B, there is shown a MIMO transmitter 102 and a MIMO receiver 104,and antennas 106, 108, 110, 112, 114 and 116. The MIMO transmitter 102may comprise a processor block 118, a memory block 120, and a signalprocessing block 122. The MIMO receiver 104 may comprise a processorblock 124, a memory block 126, and a signal processing block 128. Thereis also shown a wireless channel comprising communication paths h₁₁,h₁₂, h₂₂, h₂₁, h_(2 NTX), h_(1 NTX), h_(NRX 1), h_(NRX 2), h_(NRX NTX),where h_(mn) may represent a channel coefficient from transmit antenna nto receiver antenna m. There may be N_(TX) transmitter antennas andN_(RX) receiver antennas. There is also shown transmit symbols x₁, x₂and x_(NTX), and receive symbols y₁, y₂ and y_(NRX).

The MIMO transmitter 102 may comprise suitable logic, circuitry and/orcode that may be enabled to generate transmit symbols x_(i) iε{1, 2, . .. N_(TX)} that may be transmitted by the transmit antennas, of which theantennas 106, 108 and 110 may be depicted in FIG. 1B. The processorblock 118 may comprise suitable logic, circuitry, and/or code that maybe enabled to process signals. The memory block 120 may comprisesuitable logic, circuitry, and/or code that may be enabled to storeand/or retrieve information for processing in the MIMO transmitter 102.The signal processing block 122 may comprise suitable logic, circuitryand/or code that may be enabled to process signals, for example inaccordance with one or more MIMO transmission protocols. The MIMOreceiver 104 may comprise suitable logic, circuitry and/or code that maybe enabled to process the receive symbols y_(i) iε{1, 2, . . . N_(RX)}that may be received by the receive antennas, of which the antennas 112,114 and 116 may be shown in FIG. 1B. The processor block 124 maycomprise suitable logic, circuitry, and/or code that may be enabled toprocess signals. The memory block 126 may comprise suitable logic,circuitry, and/or code that may be enabled to store and/or retrieveinformation for processing in the MIMO receiver 104. The signalprocessing block 128 may comprise suitable logic, circuitry and/or codethat may be enabled to process signals, for example in accordance withone or more MIMO protocols. An input-output relationship between thetransmitted and the received signal in a MIMO system may be written as:y=Hx+nwhere y=[y₁, y₂, . . . y_(NRX)]^(T) may be a column vector with N_(RX)elements, .^(T) may denote a vector transpose, H=[h_(ij)]: iε{1, 2, . .. N_(RX)}; jε{1, 2, . . . N_(TX)} may be a channel matrix of dimensionsN_(RX) by N_(TX), x=[x₁, x₂, . . . x_(NTX)]^(T) is a column vector withN_(TX) elements and n is a column vector of noise samples with N_(RX)elements.

The system diagram in FIG. 1B may illustrate an exemplary multi-antennasystem as it may be utilized in a Universal Mobile TelecommunicationSystem (UMTS) Long-Term Evolution (LTE) system. Over each of the N_(TX)transmit antennas, a symbol stream, for example x₁(t) over antenna 106,may be transmitted. A symbol stream, for example x₁(t), may comprise oneor more symbols, wherein each symbol may be modulated onto a differentsub-carrier. OFDM systems may generally use a relatively large number ofsubcarriers in parallel, for each symbol stream. For example, a symbolstream x₁(t) may comprise symbols on carriers f_(m): mε{1, 2, . . . M},and M may be a subset of the FFT size that may be utilized at thereceiver. For instance, with FFT sizes of N, N>M and may createguard-tones that may allow utilization of variable bandwidth whendeployed. The M sub-carriers may comprise a symbol stream x₁(t), forexample, that may occupy a bandwidth of a few kilohertz to a fewmegahertz. Common bandwidth may be between 1 MHz and up to 100 MHz, forexample. Thus, each symbol stream may comprise one or more sub-carriers,and for each sub-carrier a wireless channel may comprise multipletransmission paths. For example, a wireless channel h₁₂ from transmitantenna 108 to receive antenna 112, as illustrated in the figure, may bemulti-dimensional. In particular, the wireless channel h₁₂ may comprisea temporal impulse response, comprising one or more multipathcomponents. The wireless channel h₁₂ may also comprise a differenttemporal impulse response for each sub-carrier f_(m) of the symbolstream, for example x₂(t). Finally, the wireless channels as illustratedin FIG. 1B depict a spatial dimension of the wireless channel becausethe transmitted signal from each transmit antenna may be receiveddifferently at each receiver antenna. Thus, a channel impulse responsemay be measured and/or estimated for each sub-carrier.

FIG. 2 is a diagram illustrating an exemplary OFDM symbol stream, inaccordance with an embodiment of the invention. Referring to FIG. 2,there is shown time-frequency axes 210; a symbol 0 comprising a cyclicprefix CP(0) 202 a, an Inverse Fast Fourier Transform (IFFT) symbol lessCP(0) (IFFT(0)) 202 b, and a cyclic prefix CP(0) 202 c, at frequency f1;a symbol 1 comprising a cyclic prefix CP(1) 204 a, an IFFT symbol lessCP(1) (IFFT(0)) 204 b, and a cyclic prefix CP(1) 204 c, at frequency f1.The IFFT(0) 202 b and the CP(0) 202 c may together form a complete IFFTsymbol for time domain symbol 0 at frequency f1. The CP(0) 202 a may besubstantially similar to CP(0) 202 c. Similarly, the IFFT(1) 204 b andthe CP(1) 204 c may together form a complete IFFT symbol for time domainsymbol 1 at f1, and CP(1) 202 a may be substantially similar to CP(1)202 c. Similarly, there is shown a symbol 0 comprising a cyclic prefixCP(0) 206 a, an Inverse Fast Fourier Transform (IFFT) symbol less CP(0)(IFFT(0)) 206 b, and a cyclic prefix CP(0) 206 c, at frequency f2. Thereis also shown a symbol 1 comprising a cyclic prefix CP(1) 208 a, an IFFTsymbol less CP(1) (IFFT(0)) 208 b, and a cyclic prefix CP(1) 208 c, atfrequency f2. There is also shown an FFT input window 214 (dashed line),a guard time Δt_(g), a frequency offset Δf, and a slot marker 208. AnLTE slot structure, for example, may comprise 3, 6, or 7 OFDM symbolsper slot (two of which may be illustrated in FIG. 2) in the time domain.

To generate an Orthogonal Frequency Division Multiplexing (OFDM) symbol,an output of an IFFT comprising of IFFT(0) 202 b and CP(0) 202 c may beused to generate CP(0) 202 a from CP(0) 202 c, and append it to IFFT(0)202 b. The cyclic prefix CP(0) 202 may be utilized to avoid inter-symbolinterference at an OFDM receiver, in the presence of multi-pathpropagation in the wireless channel.

At an OFDM receiver, for example MIMO receiver 104, a sampled inputsignal may be processed for each received symbol, for example over anFFT input window 214. In order to decode the received symbols, it may bedesirable that the FFT input window 214 may be located in a time domainsymbol time slot, for example in time domain symbol 0. In particular, itmay be desirable that the FFT input window 214 may not extend into aneighboring symbol, to avoid inter-symbol interference. Furthermore, itmay be desirable that the FFT input window 214 may not overlap multiplesymbols in the frequency domain. Thus, the slot marker may indicate thebeginning of a slot, for example time domain symbol slot 0, asillustrated in FIG. 2. The slot marker 208 together with Δt_(g) maydefine the position of the FFT input window 214 within a symbol slot inthe time domain. Similarly, a frequency carrier, for example f1 or f2,together with a frequency offset Δf may determine the location of theFFT input window 214 in the frequency domain. In most instances, to keepinterference due to the multipath channel as low as possible at thereceiver, it may be desirable to keep Δt_(g) and Δf small.

Thus, it may be desirable to acquire frequency carrier, for example f1,tracking, and maintain frequency tracking as the frequency may drift,for example, because of changes in propagation due to mobility. In someinstances, this may be combined with other frequency acquisition andtracking processes. In many instances, coarse frequency synchronizationvia the Primary Synchronization Signal (PSS) and the SecondarySynchronization Signal (SSS) may be obtained initially. Fine frequencytracking may be acquired by a frequency acquisition and tracking system,which may exploit reference signals (RS) embedded in an OFDM signal.Reference symbols may be known symbols that may be transmitted accordingto a known pattern over the time, frequency and spatial resources in anOFDM system. In other words, reference symbols may be transmitted atknown timing instances, on known OFDM carriers over certain antennas. Bydecoding and processing RS symbols, the receiver may determine correcttiming and frequency information, for example, through coherentdemodulation. RS symbols may be transmitted from each antenna in amultiple antenna OFDM system.

In the Enhanced Universal Terrestrial Radio Access (EUTRA) interface, RSsymbols may be generated based on cell-specific hopping pattern, and maycomprise pseudo-noise (PN) covered sequences of Reference symbols. Inaccordance with an embodiment of the invention, the RS tone spacing maybe 6 carriers, per transmit antenna, for example. In accordance withvarious embodiments of the invention, the RS tone spacing may be 2, or 4carriers, for example. The RS sequence may not be known to the mobileterminal (user equipment, UE) during initial acquisition, for examplethrough the synchronization signals. In some instances, after acquiringthe primary synchronization signal (PSS) and the secondarysynchronization signal (SSS), the UE may have obtained the cell-specifichopping pattern for the RS symbols, and the PN covering sequence. Thisinformation may be used to obtain coarse frequency information. Inaccordance with various embodiments of the invention, the RS symbols maythen be decoded in a frequency acquisition and tracking block to providefine frequency tracking.

FIG. 3 is a diagram of an exemplary OFDM frequency acquisition andtracking system, in accordance with an embodiment of the invention.Referring to FIG. 3, there is shown a common receiver part 342, and afrequency part 340. The frequency part 340 may comprise an RS frequencydiscriminator 302, an adder 304, a delay block 306, an integrator 308,and a threshold block 310. There is also shown an RS set input, an errorsignal, an accumulator signal ff_loop_accum, a threshold input signal, areset control signal reset_cntrl, and an output signal tcxo_accum. Thecommon receiver part 342 may comprise a timing generator 312, an RSextraction module or circuit 314, a channel estimation block 316, areceiver operations block (RXCVR) 318, a fast Fourier transform (FFT)block 320, a buffering block 330, a sampling bandwidth (BW) filter 332,an analog-to-digital block 334, a master timer 336, and aTemperature-Controlled crystal Oscillator (TCXO) 338. There is alsoshown an RF filter input, a master timer output, a slot timing inputfrom PSS, an RS set output, a tcxo_accum signal, an rs_strb signal, anda slot_strb signal.

The frequency part 340 may comprise suitable logic, circuitry and/orcode that may be enabled to extract frequency information by processingan RS set of signals, which may generate an output tcxo_accum that maycontrol the TCXO 338, for example. The RS frequency discriminator 302may comprise suitable logic, circuitry and/or code that may be enabledto compare the frequency of the RS set input signal with, for example,an input clock signal and may generate an error signal f_(k). The adder304 may comprise suitable logic, circuitry and/or code that may beenabled to generate a weighted sum signal at its output, from aplurality of inputs. The delay block 306 may comprise suitable logic,circuitry and/or code that may be enabled to delay an input signal by acertain time interval, for example one or more sample periods. The delayblock 306 may comprise suitable logic, circuitry and/or code that may beenabled to delay an input signal by a certain time interval, for exampleone or more sample periods. The integrator 308 may comprise suitablelogic, circuitry and/or code that may be enabled to generate an outputthat may be the integration of one or more input signals, and theintegrator 308 may be reset by the reset_cntrl signal. The thresholdblock 310 may comprise suitable logic, circuitry and/or code that may beenabled to compare a threshold input signal with the ff_loop_accum inputsignal, and generate output signals reset_cntrl and tcxo_accum. Forexample, when the ff_loop_accum signal may exceed the threshold level,the reset_cntrl signal may activate and reset, for example, theintegrator 308.

The common receiver part 342 may comprise suitable logic, circuitryand/or code that may be enabled to receive radio frequency signals, andprocess these signals. Processing may comprise FFT computation, RSsymbol extraction, channel estimation and other receiver signalprocessing. The timing generator 312 may comprise suitable logic,circuitry and/or code that may be enabled to generate timing signals forRS extraction, rs_strb, and slot timing, slot_strb. The signal slot_strbmay be used to control FFT timing in the buffering block 330, forexample. The module or circuit 314 may comprise suitable logic,circuitry and/or code that may be enabled to extract the RS symbols fromthe FFT module or circuit 320 output. The channel estimation module orcircuit 316 may comprise suitable logic, circuitry and/or code that maybe enabled to estimate the wireless channel response for RS symbols,which may be desirable for receiver operations. The receiver operationsmodule or circuit (RXCVR) 318 may comprise suitable logic, circuitryand/or code that may be enabled to measure and/or verify performanceduring receiver operations. The fast Fourier-transform (FFT) module orcircuit 320 may comprise suitable logic, circuitry and/or code that maybe enabled to generate a Fast Fourier Transform for an input signal. Thebuffering module or circuit 330 may comprise suitable logic, circuitryand/or code that may be enabled to interface with, for example, the FFTengine. The buffering module or circuit 330 may assist in dedicatedprocesses, measurement processes, multimedia broadcast multicastservices (MBMS), and/or SSS processing for RS PN sequence determination.In some instances, each of the processes may be performed in parallel.The sample BW filter 332 may comprise suitable logic, circuitry and/orcode that may be enabled to filter the signal at its input, and generatean output signal with limited bandwidth. The analog-to-digital (A2D)module or circuit 334 may comprise suitable logic, circuitry and/or codethat may be enabled to receive an analog RF-filtered signal and convertit to a digital signal representation at the output, with an arbitrarynumber of bits. The master timer 336 may comprise suitable logic,circuitry and/or code that may be enabled to provide basic timingfunctionality in the receiver. In some instances, the master timer 336may count over 10 ms periods, and may be clocked at 30.72 MHz, forexample. The master counter may comprise a slot counter, and a samplecounter. The TCXO 338 may comprise suitable logic, circuitry and/or codethat may be enabled to generate a variable frequency output signal, as afunction of an input signal, for example a voltage.

The common receiver part 342 may receive radio frequency signals, andprocess these signals. Processing may comprise FFT computation, RSsymbol extraction, channel estimation and other receiver signalprocessing. Some frequency aspects of the common receiver part 342 maybe controlled by the frequency part 340. For example, the receiversubcarrier/carrier frequency, for example f1 and/or f2 as illustrated inFIG. 2, may be determined via the TCXO 338.

The RS frequency discriminator 302 may receive at its input, a set of RSsymbols, which may be extracted in the RS extraction module or circuit314 to determine frequency information about the RS carrying-carrier.The output of the RS frequency discriminator 302 may be communicativelycoupled to a first input of the adder 304. A second input of the adder304 may be coupled to the output of the delay module or circuit 306, andthe output of the adder 304 may be a weighted sum of its inputs. Thedelay module or circuit 306 may delay an input signal by a certain timeinterval, for example one or more sample periods. The delay module orcircuit 306 may provide appropriate delay for the feedback signalprovided to the adder 304 from the integrator 308.

An output of the adder 304, ff_loop_accum, may be communicativelycoupled to a first input of the threshold module or circuit 310, and afirst input of the integrator 308. A second input of the thresholdmodule or circuit 310 may be coupled to a threshold-level definingsignal. The threshold module or circuit 310 may compare a thresholdsignal with the ff_loop_accum signal, and generate output signalsreset_cntrl and tcxo_accum. For example, when the ff_loop_accum signalmay exceed the threshold level, the reset_cntrl signal may activate andreset, for example, the integrator 308. In accordance with an embodimentof the invention, the tcxo_accum signal, may increase at a rate that isa function of Δf, and may thus allow information about Δf to becommunicated to, for example, the TCXO 338, which in turn may controlthe FFT input window's position in the frequency domain. A second inputto the integrator 308 may be set to a known, constant level, for examplezero. The integrator 308 may integrate one or more input signals, andthe integrator 308 may be reset by the reset_cntrl signal.

The analog-to-digital (A2D) module or circuit 334 may receive an analogRF-filtered signal and convert it to a digital signal representation atthe output, with an arbitrary number of bits. The A2D 334 output may becommunicatively coupled to an input of the sample BW filter 332. Thesample BW filter 332 may filter the signal at its input, and generate anoutput signal with limited bandwidth and/or attenuate certain frequencybands. The output of the sample BW filter 332 may be communicativelycoupled to a first input of the buffering module or circuit 330. Asecond input to the buffering module or circuit 330 may becommunicatively coupled to the output signal slot_strb from the timinggenerator 312. The buffering module or circuit 330 may interface with,for example, the FFT engine. The buffering module or circuit 330 mayassist in dedicated processes, measurement processes, multimediabroadcast multicast services (MBMS), and/or SSS processing for RS PNsequence determination. In some instances, each of the processes may beperformed in parallel. The output of the buffering module or circuit 330may be communicatively coupled to the FFT module or circuit 320.

The FFT module or circuit 320 may generate a Fast Fourier Transform foran input signal communicatively coupled from the buffering module orcircuit 330. Similar to the buffering module or circuit 330, the FFTmodule or circuit 320 may assist in signal processing for dedicatedprocesses, measurement processes, multimedia broadcast multicastservices (MBMS), and/or SSS processing for radio time framing andhopping pattern determination. A first output of the FFT module orcircuit 320 may be communicatively coupled to a first input of the RSextraction module or circuit 314. The RS extraction module or circuit314 may extract the RS symbols from the FFT module or circuit 320output. In some instances, it may be desirable to use a generatedhopping sequence from the demodulated base station signal and/orpseudo-noise (PN) covering for RS decoding. The RS symbols extracted andoutput at the RS extraction module or circuit 314 may be communicativelycoupled to the input of the frequency part 340, and a channel estimationmodule or circuit 316. The hopping pattern may be fed to the RSextraction module or circuit 314 via the rs_hopping_pattern signal on asecond input, as illustrated in FIG. 3. The RS extraction module orcircuit 314 timing may be controlled via a third input signal rs_strb,communicatively coupled to an output of the timing generator 312.

The timing generator 312 may generate timing signal for RS extraction,rs_strb, and slot timing, slot_strb. The signal slot_strb may be used tocontrol FFT timing in the buffering module or circuit 330. The timinggenerator 312 may generate the output timing signals from a function ofthe master timer input signal, slot timing (PSS), for timing correctionsand tracking. The master timer input signal may be communicativelycoupled to the master timer 336 output. The master timer 336 may providebasic timing functionality in the receiver. In some instances, themaster timer 336 may count over 10 ms periods, and may be clocked at30.72 MHz, for example. The master counter may comprise a slot counter,and a sample counter. The input to the master timer 336 may be providedby an operating RF crystal, the temperature-controlled crystaloscillator (TCXO) 338, for example. The TCXO 338 may be communicativelycoupled to the threshold module or circuit 310 via the tcxo_accumsignal.

The channel estimation module or circuit 316 may estimate the wirelesschannel response for RS symbols, which may be desirable for receiveroperations. The channel estimation output may be communicatively coupledto the RXCVR 318. The RXCVR 318 may measure and/or verify receiverperformance functionality.

FIG. 4 is a flow chart illustrating an exemplary frequency acquisitionand tracking process, in accordance with an embodiment of the invention.After initialization in step 402, the primary and secondarysynchronization signals, PSS and SSS respectively, may be decoded instep 404. The decoding of the PSS and SSS may provide coarse frequencyinformation for frame and slot synchronization, for example. Using thefrequency information, the FFT module or circuit 320, for example, maygenerate an FFT of a received signal. From this FFT, the RS extractionmodule or circuit 314, for example, may extract an RS set in the RSextraction module or circuit 314. This RS set may be fed to thefrequency part 340. In step 340, the frequency part 340 may track Δfusing the feedback loop depicted in FIG. 3. Since the output signaltcxo_accum may be generated as a function of Δf, the tcxo_accum outputsignal from the threshold module or circuit 310 may carry informationabout Δf to the TCXO 338. Thus, the output signal tcxo_accum may enabletracking of the carrier frequency, and adjust the frequency of thereceiver, for example in the master timer 336 via the TCXO 338. In step410, the master timer 336 may adjust the frequency based on the inputsignal tcxo_accum. The adjusted and tracked frequency may becommunicatively coupled to the master timer 336, so that the carrierfrequency for the FFT input window 214 may be adjusted correspondingly.

In accordance with an embodiment of the invention, a method and systemfor an RS frequency control loop for TCXO synchronization and trackingmay comprise tracking a carrier frequency in an Orthogonal FrequencyDivision Multiplexing (OFDM) signal based on at least a reference symbolset, as illustrated in FIG. 2 and FIG. 3. A receiver frequency may beadjusted based on at least the tracked carrier frequency.

The carrier frequency may be tracked by generating an output signal as afunction of a frequency offset Δf in a frequency discrimination feedbackloop, for example the frequency part 340. The reference symbol (RS) setmay be generated in an RS extraction module or circuit 314, from atleast a fast Fourier transform of the received OFDM signal, generated inthe FFT module or circuit 320, for example. The receiver frequency maybe finely adjusted, whereby the receiver frequency may be adjustedcoarsely prior to the fine adjustment. The coarse receiver frequencyadjustment may be based on processing at least a primary synchronizationsignal and a secondary synchronization signal, as described for FIG. 2and FIG. 3. The reference symbol set may comprise a plurality oftime-frequency slots. The plurality of time-frequency slots may changeaccording to a time-frequency shift and pseudo-noise sequence thatmodulates the reference symbols. The OFDM signal may conform with aUniversal Mobile Telecommunications Standards (UMTS) long-term evolution(LTE) signal. The adjustment of the receiver frequency may be controlledvia an oscillator, for example the TCXO 338.

Another embodiment of the invention may provide a machine-readableand/or computer-readable storage and/or medium, having stored thereon, amachine code and/or a computer program having at least one code sectionexecutable by a machine and/or a computer, thereby causing the machineand/or computer to perform the steps as described herein for a methodand system for an RS frequency control loop for TCXO synchronization andtracking.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

1. A method for processing communication signals, the method comprising:tracking a carrier frequency in an Orthogonal Frequency DivisionMultiplexing (OFDM) signal based on at least a reference symbol set; andadjusting a receiver frequency based on at least said tracked carrierfrequency, wherein said reference symbol set comprises a plurality oftime-frequency slots, and wherein said plurality of time-frequency slotschanges according to a time-frequency shift and pseudo-noise sequencethat modulates one or more reference symbols in said reference symbolset.
 2. The method according to claim 1, comprising tracking saidcarrier frequency by generating an output signal as a function of afrequency offset.
 3. The method according to claim 2, comprisinggenerating said output signal in a frequency discrimination feedbackloop.
 4. The method according to claim 1, comprising generating saidreference symbol (RS) set in an RS extraction module or circuit, from atleast a fast Fourier transform of said received OFDM signal.
 5. Themethod according to claim 1, comprising finely adjusting said receiverfrequency.
 6. The method according to claim 5, comprising coarselyadjusting said receiver frequency prior to said fine adjustment.
 7. Themethod according to claim 6, comprising generating said coarse receiverfrequency adjustment based on processing at least a primarysynchronization signal and a secondary synchronization signal.
 8. Themethod according to claim 1, wherein said OFDM signal conforms with aUniversal Mobile Telecommunications Standards (UMTS) long-term evolution(LTE) signal.
 9. The method according to claim 1, comprising controllingadjustment of said receiver frequency via an oscillator.
 10. A systemfor processing communication signals, the system comprising: one or morecircuits in a receiver operable to, at least: track a carrier frequencyin an Orthogonal Frequency Division Multiplexing (OFDM) signal, whereinsaid tracking is based on at least a reference symbol set; and adjust areceiver frequency based on at least said tracked carrier frequency,wherein said reference symbol set comprises a plurality oftime-frequency slots, and wherein said plurality of time-frequency slotschanges according to a time-frequency shift and pseudo-noise sequencethat modulates one or more reference symbols in said reference symbolset.
 11. The system according to claim 10, wherein said one or morecircuits track said carrier frequency by generating an output signal asa function of a frequency offset.
 12. The system according to claim 11,wherein said one or more circuits generate said output signal in afrequency discrimination feedback loop.
 13. The system according toclaim 10, wherein said one or more circuits comprises an RS extractionmodule operable to generate said reference symbol (RS) set from at leasta fast Fourier transform of said received OFDM signal.
 14. The systemaccording to claim 10, wherein said one or more circuits finely adjustsaid receiver frequency.
 15. The system according to claim 14, whereinsaid one or more circuits coarsely adjust said receiver frequency priorto said fine adjustment.
 16. The system according to claim 15, whereinsaid one or more circuits generate said coarse receiver frequencyadjustment based on processing at least a primary synchronization signaland a secondary synchronization signal.
 17. The system according toclaim 10, wherein said OFDM signal conforms with a Universal MobileTelecommunications Standards (UMTS) long-term evolution (LTE) signal.18. The system according to claim 10, wherein said one or more circuitscomprises and oscillator operable to control adjustment of said receiverfrequency.